micro-groove

Ballistic fingerprinting

Ballistic fingerprinting, a sub-category of firearms examination, is a forensic method that is intended to help find the gun that was used in a crime by matching the bullet's striations (or striae) with the rifled barrel through which it was fired, or by matching marks on the cartridge case to marks in the chamber and breech. The technique is part of the science of forensic ballistics, and it is an application of toolmark identification. The term ballistic fingerprinting, a comparison to the use of fingerprints in forensics, is more commonly encountered by the public.

Applications

The basic application of the technique is for forensic firearms examiners to examine the bullet or cartridge case evidence and reach a conclusion about common origin (typically restricted to "yes," "no" or "maybe"). The simplest considerations, class characteristics, are the gross differences; a 10 mm bullet, for example, could not have been fired from a 9 mm barrel, nor could a .357 Magnum cartidge have been fired from a .38 Specialrevolver (although a .38 Special could be fired from a .357 Magnum revolver). The rifling in the barrel also varies among manufacturers and models, in number and shape of the grooves, twist rate, and direction. Colt, for example, traditionally uses a left-hand twist, while Smith and Wesson uses a right hand twist; a current production M16 rifle uses a 1 in 7 inch twist, while most civilian AR-15s and the current Mini-14 use a 1 in 9 inch twist. Marlin Firearms use a distinctive 16-groove Micro-Groove rifling in many of their firearms, while the M1903 Springfield rifle had two, four, or six grooves depending on the manufacturer. Examiners can often quickly narrow down the make and model based on these variations; while they cannot say with a high degree of certainty that "gun A" fired a given bullet or case based on this information alone, they may be able to say that "gun A" did not fire a given bullet or case.

As all manufactured items have inevitable variations, it is often possible for a forensic firearms examiner to match a bullet or cartridge case to a particular firearm based on these variations. Most often these are due to marks left by a machining process, which can leave shallow impressions in the metal which are rarely completely polished out. Wear due to use will also cause each firearm to acquire distinct characteristics over time, though this same process will also alter the "fingerprint" of the firearm. If recovered, cartridge cases are often easier to identify than bullets. First, the parts of a firearm that produce marks on cartridge cases are less subject to long-term wear, and second, bullets are often severely deformed on impact, destroying much of the markings they acquire.

Connecting crimes

The most common current use of computer-based ballistic fingerprinting is to match cases or bullets found at various crime scenes to one another, to provide connections for police to use in investigations. A human firearms examiner must examine the actual evidence for a conclusive match, but even a probable match can help police look for additional evidence in areas they would not otherwise have considered.

Ballistic fingerprint databases

Some localities, particularly Maryland, have attempted to build up a large database of "fingerprints"; in the case of the Maryland law, all new firearms sales must provide a fired case from the firearm in question to the Maryland State Police, who photograph it and log the information in a database. The Maryland State Police wrote a report critical of the program and asking the Maryland General Assembly to disband it, since it was expensive and had not contributed to solving a single crime. Subsequently however the database did provide evidence used to obtain a murder conviction.

A California Department of Justice survey, using 742 guns used by the California Highway Patrol as a test bed, showed very poor results; even with such a limited database, less than 70% of cases of the same make as the "fingerprint" case yielded the correct gun in the top 15 matches; when a different make of ammunition was used, the success rate dropped to less than 40%. California has passed a bill AB 1471 which requires all new models of handguns to be equipped with microstamping technology by 2010.

Alteration of fingerprints

Although every rifled barrel leaves a unique "fingerprint" on a bullet fired through it so that comparisons can sometimes be performed, it is extremely easy to permanently or temporarily modify a ballistic fingerprint at will. Also, polygonal rifling may leave striae that are difficult to match to a particular barrel, such that some police agencies have forbidden the use of weapons with such rifling in order to match accurately after the fact just which gun fired which bullet.

There is also a possibility that the owner of a weapon might replace the barrel (if it is worn out, for example), which will change the fingerprint. As mentioned above, use will add wear marks to the firearm, which will alter the fingerprint by obscuring some existing marks, and creating new ones. Temporary changes in fingerprinting are also possible. Specifically, it is possible to buy a replacement barrel for most firearms--often for as little as US$20 for a used part. It is possible to replace the barrel in many firearms, especially semi-automatic handguns, in minutes, with no tools. This barrel may be used in a crime, then removed just as quickly.

Shotguns

Ballistic fingerprinting of bullets does not work at all with firearms such as shotguns that fire shot-containing cartridges. In many cases the shot rides inside a plastic sleeve that prevents it from ever touching the barrel, and even in cases where the shot does touch the barrel, the random movement of the shot down the barrel will not leave any consistent marks.